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Characterization by electron paramagnetic resonance techniques of several variants of Anabaena flavodoxin, where the naturally occurring FMN cofactor is substituted by different analogs, makes it possible to improve the details of the spin distribution map in the isoallosazine ring in its semiquinone state. The analyzed variants were selected to monitor the effects of intrinsic changes in the flavin ring electronic structure, as well as perturbations in the apoflavodoxin-flavin interaction, on the spin populations. When these effects were analyzed together with the functional properties of the different flavodoxin variants, a relationship between spin population and biochemical parameters, as the reduction potential, could be envisaged.

The magnetic response of a Kramers doublet is analyzed in a general case taking into account only the formal properties derived from time reversal operation. It leads to a definition of a matrix G (gyromagnetic matrix) whose expression depends on the chosen reference frame and on the Kramers conjugate basis used to describe the physical system. It is shown that there exists a reference frame and a suitable Kramers conjugate basis that gives a diagonal form for the G-matrix with all non-null elements having the same sign. A detailed procedure for obtaining this canonical expression of G is presented when the electronic structure of the KD is known regardless the level of the used theory. This procedure provides a univocal way to compare the theoretical predictions with the experimental results obtained from a complete set of magnetic experiments. In this way the problems arising from ambiguities in the g-tensor definition are overcome. This procedure is extended to find a spin-Hamiltonian suitable for describing the magnetic behavior of a pair of weakly coupled Kramers systems in the multispin scheme when the interaction between the two moieties as well as the individual Zeeman interaction are small enough as compared with ligand field splitting. Explicit relations between the physical interaction and the parameters of such a spin-Hamiltonian are also obtained.

Transition metal ions with long-lived spin states represent minimum size magnetic bits. Magnetic memory has often been associated with the combination of high spin and strong uniaxial magnetic anisotropy. Yet, slow magnetic relaxation has also been observed in some Kramers ions with dominant easy-plane magnetic anisotropy, albeit only under an external magnetic field. Here we study the spin dynamics of cobalt(II) ions in a model molecular complex. We show, by means of quantitative first-principles calculations, that the slow relaxation in this and other similar systems is a general consequence of time-reversal symmetry that hinders direct spin-phonon processes regardless of the sign of the magnetic anisotropy. Its magnetic field dependence is a subtle manifestation of electronuclear spin entanglement, which opens relaxation channels that would otherwise be forbidden but, at the same time, masks the relaxation phenomenon at zero field. These results provide a promising strategy to synthesize atom-size magnetic memories. © 2014 Macmillan Publishers Limited. All rights reserved.

In this contribution we present the study of the thermal dependence of the ENDOR spectra of flavodoxin at low temperatures which reveals the dynamics of the methyl groups bound to the flavin moiety in flavoproteins. The methyl groups behave as quantum rotors locked by a deep rotational well and undergoing a tunneling process. At room temperature, methyl rotors are locked and the hopping motion is slow. This picture of the dynamics of the methyl groups of the flavin ring is quite different from the one usually accepted and has relevant consequences on the understanding of the mechanisms of flavoproteins.

A major challenge for realizing quantum computation is finding suitable systems to embody quantum bits (qubits) and quantum gates (qugates) in a robust and scalable architecture. An emerging bottom-up approach uses the electronic spins of lanthanides. Universal qugates may then be engineered by arranging in a molecule two interacting and different lanthanide ions. Preparing heterometallic lanthanide species is, however, extremely challenging. We have discovered a method to obtain [LnLn'] complexes with the appropriate requirements. Compound [CeEr] is deemed to represent an ideal situation. Both ions have a doubly degenerate magnetic ground state and can be addressed individually. Their isotopes have mainly zero nuclear spin, which enhances the electronic spin coherence. The analogues [Ce2], [Er2], [CeY], and [LaEr] have also been prepared to assist in showing that [CeEr] meets the qugate requirements, as revealed through magnetic susceptibility, specific heat, and EPR. Molecules could now be used for quantum information processing.

The thermally stable, paramagnetic Pt(III) complex [PtI2(IPr) 2][BArF] has been prepared by oxidation of the Pt(ii) complex [PtI2(IPr)2] with iodine in the presence of NaBArF. X-ray crystallographic studies revealed the mononuclear nature of this species with a square-planar geometry. EPR and DFT studies pointed out to a metal-centred radical. © The Royal Society of Chemistry.

Homoleptic organocobalt(III) compounds with formula [NBu4][CoIII(C6X5)4] [X = F (3), Cl (4)] were obtained in reasonable yields by chemical oxidation of the corresponding divalent species [NBu4]2[CoII(C6X5)4] [X = F (1), Cl (2)]. The [CoIII(C6X5)4]-/[CoII(C6X5)4]2- couples are electrochemically related by quasi-reversible, one-electron exchange processes at moderate potential: E1/2 = -0.29 (X = F) and -0.36 V (X = Cl) versus saturated calomel electrode. The [CoIII(C6X5)4]- anions in salts 3 and 4 show an unusual square-planar geometry as established by single-crystal X-ray diffraction methods. According to their stereochemistry, these CoIII derivatives (d6) are paramagnetic non-Kramers systems with a large zero-field splitting contribution and no observable electron paramagnetic resonance (EPR) spectrum. The thermal dependence of their magnetic susceptibilities can be explained in terms of a spin-Hamiltonian formalism with S = 1 ground state (intermediate spin) and substantial spin-orbit contribution. The magnetic properties of the square-planar d7 parent species [NBu4]2[CoII(C6X5)4] were also thoroughly studied both at microscopic (EPR) and macroscopic levels (alternating current and direct current magnetization measurements). They behave as S = 1/2 (low spin) systems with mainly (dz2 )1 electron configuration and a certain degree of s-orbital admixture that has been quantified. The electronic structures of all four open-shell [Co(C6X5)4]q- compounds (q = 1, 2) accounting for their respective magnetic properties are based on a common orbital energy-level diagram.

A complete study was made of the stabilization of gamma-irradiated ultrahigh molecular weight polyethylene (UHMWPE) using arc-discharge multi-walled carbon nanotubes (MWCNTs) as inhibitors of the oxidative process. MWCNTs were efficiently incorporated into the polymer matrix by ball milling and thermo-compression processes at concentrations up to 5 wt% and subsequently gamma irradiated at 90 kGy. Raman spectroscopy demonstrated the generation of radicals on the walls of the MWCNTs and that the G/D ratio was altered by their generation. The same spectra showed interactions between the polymer chains as a series of shifts are observed in the UHMWPE bands. The effect of the MWCNTs as inhibitors for the oxidative process of the UHMWPE was evaluated by means of Electron Spin Resonance (ESR) and Fourier Transformed Infrared Spectroscopy (FTIR). ESR detection of the radiation-induced radicals proved the radical scavenger behaviour of MWCNTs. FTIR measurements were performed to ascertain the influence of the irradiation and of the accelerated ageing protocol in the oxidation index of the polymer and the composites. The results pointed to the positive contribution of the MWCNTs in increasing the oxidative stability of the composite when compared to pure UHMWPE. A comparison is made between composites obtained using MWCNTs produced by the carbon vapour deposition and arc-discharge methods. © 2013 Springer Science+Business Media New York.

The impact of chromium (Cr) on fish health has been the subject of numerous investigations, establishing a wide spectrum of toxicity, attributed particularly to the hexavalent form [Cr(VI)]. However, reports on the simultaneous assessment of Cr toxicity in fish and its toxico-kinetics, namely involving metal speciation, are scarce. Therefore, keeping in view the understanding of the mechanisms of Cr(VI) toxicity, this work intended to detect the formation of paramagnetic Cr species in liver of Anguilla anguilla following short-term dichromate(VI) intraperitoneal treatment (up to 180 min), assessing simultaneously the pro-oxidant properties. The formation of Cr(V) and Cr(III) was examined by electron paramagnetic resonance (EPR), as an innovative approach in the context of fish toxicology, and related with the levels of total Cr. Cr(V) was successfully detected and quantified by EPR spectrometry, showing a transient occurrence, mostly between 15 and 90 min post-injection, with a peak at 30 min. The limitations of EPR methodology towards the detection and quantification of Cr(III) were confirmed. Although Cr(VI) exposure induced the antioxidant system in the eel's liver, the oxidative deterioration of lipids was not prevented. Overall, the results suggested that Cr(V), as a short-lived species, did not appear to be directly and primarily responsible for the cellular damaging effects observed, since stress responses persisted up to the end of exposure regardless Cr(V) drastic decay. Though further research is needed, ROS mediated pathways (suggested by superoxide dismutase and catalase activity induction) and formation of Cr(III) complexes emerged as the most plausible mechanisms involved in Cr(VI) toxicity. © 2012 Elsevier Inc.

A novel, vitamin E-stabilized, medical grade ultra-high molecular polyethylene, MG003 (DSM Biomedical; The Netherlands), has been very recently introduced for use in total joint replacements. This homopolymer resin features average molecular weight similar to that of conventional GUR 1050 resin (5.5-6*10 6 g/mol), but a higher degree of linearity. The aim of this study was to characterize the microstructure, thermal and thermooxidation properties as well as the mechanical behavior of this novel MG003 resin before and after gamma irradiation in air to 90 kGy. For this purpose, a combination of experimental techniques were performed including differential scanning calorimetry (DSC), thermogravimetry (TG), transmission electron microscopy (TEM), X-Ray Diffraction, electron paramagnetic resonance (EPR), and uniaxial tensile tests. As-consolidated MG003 materials exhibited higher crystalline contents (~ 62%), transition temperatures (~ 140 °C), crystal thickness (~ 36 nm), yield stress (~ 25 MPa) and elastic modulus (~ 400 MPa) than GUR 1050 controls (55%, 136 °C, 27 nm, 19 MPa, and 353 MPa, respectively). Irradiation produced similar changes in both MG003 and GUR 1050 materials, specifically increased crystallinity (63% and 60%, respectively), crystal thickness (39 nm and 30 nm), yield stress (27 MPa and 21 MPa), but, above of all, loss of elongation to breakage (down to 442 and 469%, respectively). Thermogravimetric and EPR results suggest comparable susceptibilities to oxidation for both MG003 and GUR 1050 polyethylenes. Based on the present findings, MG003 appears as a promising alternative medical grade polyethylene and it may satisfactorily contribute to the performance of total joint replacements. © 2012 Elsevier B.V.

We report here a novel effect in which the resistance of a semiconducting oxide ceramic increases on application of a small dc bias. The ceramic conducts at high temperatures by an n-type hopping mechanism. On application of a dc bias, conduction electrons are trapped at surface states and the resistance increases. On removal of the dc bias, the trapped electrons are released and the sample regains its original state. This effect is the mirror image of that seen with similar ceramics that conduct by a p-type mechanism whose resistance decreases reversibly on application of a small dc bias. These two phenomena together offer the possibility of novel switching devices and memristive applications, especially if the switching times can be reduced. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In recent years, magnetic molecular clusters have been proposed as suitable materials for the realization of the quantum computer hardware. Here, we show, via a combination of ac susceptibility, magnetization and heat capacity studies performed in the vicinity of the absolute zero, that molecular clusters containing two Tb3+ ions meet the ingredients required to implement a CNOT quantum gate. The definition of control and target qubits is based on the strong magnetic anisotropy and the magnetic inequivalence of the two ions, which has been achieved by chemically engineering dissimilar coordination spheres. The magnetic asymmetry also provides a method to realize a SWAP gate in the same cluster. Electronic paramagnetic resonance experiments confirm that CNOT and SWAP transitions are not forbidden. Although we have only considered Tb2, for which the magnetic asymmetry can be easily determined on account of its large angular momentum, the same molecular structure can be realized with other lanthanide ions. This flexibility enables a vast choice of quantum gate designs. These molecular clusters are stable in solution, which opens the possibility of depositing them onto devices able to manipulate its quantum spin state. Chemically engineered molecular quantum gates can therefore open promising avenues for the realization of scalable quantum computing architectures.

We report ac susceptibility and continuous wave and pulsed EPR experiments performed on GdW 10 and GdW 30 polyoxometalate clusters, in which a Gd3 + ion is coordinated to different polyoxometalate moieties. Despite the isotropic character of gadolinium as a free ion, these molecules show slow magnetic relaxation at very low temperatures, characteristic of single molecule magnets. For T200mK, the spin-lattice relaxation becomes dominated by pure quantum tunneling events, with rates that agree quantitatively with those predicted by the Prokof'ev and Stamp model. The sign of the magnetic anisotropy, the energy level splittings, and the tunneling rates strongly depend on the molecular structure. We argue that GdW 30 molecules are also promising spin qubits with a coherence figure of merit QM≳50. © 2012 American Physical Society.

Eliminating restraints: A trigonal-bipyramidal structure has been found to be the energetically favored geometry of the hypervalent AX 5 molecule Sb(C 6F 5) 5 in the solid state and also in fluid solution, where molecules move freely and no crystal packing effects operate (see picture).

Multi-walled carbon nanotubes (MWCNTs) were incorporated in ultrahigh molecular weight polyethylene (UHMWPE), which is a polymer used in industrial and orthopedic applications. The composites were prepared by ball milling and thermo-compression processes at concentrations up to 3 wt.% and subsequently gamma irradiated at 90 kGy. Electrical conductivity measurements showed a low percolation threshold of 0.5 wt.%. Electron spin resonance detection of the radiation-induced radicals proved the radical scavenger behavior of MWCNTs: when the nanotube concentration increased, the number of radicals generated by the gamma irradiation process decreased. Allyl radicals seem to be the radicals most affected by the presence of nanotubes in this polymeric matrix. Fourier transformed infrared spectroscopy measurements and an accelerated ageing protocol were performed to ascertain the influence of the irradiation on the oxidation index. The results pointed to the positive contribution of the MWCNTs in increasing the oxidative stability of the composite compared to pure UHMWPE. Crosslinking density induced by gamma irradiation was obtained by swelling measurements. The findings showed that, despite the radical scavenger performance, MWCNTs are capable of maintaining the efficiency of the crosslinking density, unlike the other antioxidants, which inhibit radiation crosslinking. © 2012 Elsevier Ltd. All rights reserved.

The mononuclear, five-coordinate organochromium(v) compound [NBu 4][CrO(C 6F 5) 4] (1) has been obtained as a dark red solid in moderate yield by treatment of the homoleptic organochromium(iii) derivative [NBu 4] 2[Cr(C 6F 5) 5] with NO[BF 4] in CH 2Cl 2 solution under an oxygen atmosphere. The Cr V centre in the [CrO(C 6F 5) 4] - anion shows a square-pyramidal geometry with the four C 6F 5 groups in the basal positions and the oxo ligand in the apical one (X-ray). The short Cr-O distance (153.8(2) pm) suggests a high degree of triple bond character for the chromyl unit. The EPR spectrum of 1 in solution shows an isotropic signal with g iso = 1.995(1) and a rich hyperfine structure due to coupling with the 53Cr isotope [a Cr = 46.95(4) MHz] as well as with the ortho-F and meta-F substituents of the C 6F 5 rings [a F = 4.20(2) MHz and a′ F = 2.12(2) MHz] in keeping with the presence of non-interacting, fast tumbling, paramagnetic [CrO(C 6F 5) 4] - units (d 1). In the solid state, however, both the microscopic (EPR) and macroscopic magnetic properties (isothermal magnetisation and thermal dependence of the magnetic susceptibility) suggest the existence of weak ferromagnetic interactions with T C = 0.20(2) K. Such magnetic interactions may probably be favoured by Ï€ interactions between C 6F 5 rings of neighbour [CrO(C 6F 5) 4] - units in the crystal. © 2012 The Royal Society of Chemistry.

The homoleptic, square-planar trifluoromethylplatinate(ii) compound [NBu4]2[Pt(CF3)4] (1) reacts with SOCl2 undergoing oxidative addition of a S-Cl bond to give the octahedral species [NBu4]2[trans-Pt(CF3) 4Cl(SOCl)] (4), that contains the unusual chlorosulfinyl ligand. Compound 4 readily evolves into the dichloro-derivative [NBu4] 2[cis-Pt(CF3)4Cl2] (2), whereby the >Pt(CF3)4> unit undergoes a stereochemical rearrangement from an initial square-planar (equatorial) geometry to a final sawhorse disposition within the global Pt octahedral environment. Compound 2 is more reactive than the corresponding [NBu4]2[trans- Pt(CF3)4Cl2] (3) stereoisomer and thus affords the trichloro-derivative [NBu4]2[fac-Pt(CF 3)3Cl3] (5) by treatment with the stoichiometrically required amount of HCl(aq). Stereoisomer [NBu 4]2[mer-Pt(CF3)3Cl3] (6) has been obtained by oxidative addition of Cl2 to the organoplatinum(ii) precursor [NBu4]2[Pt(CF 3)3Cl]. All the synthetic procedures described here proceed in high yields and in a stereoselective manner. The optical properties of the cis-/trans-[Pt(CF3)4Cl2]2- and fac-/mer-[Pt(CF3)3Cl3]2- diastereomeric couples (diffuse reflectance) as well as the solid-state geometries of the latter couple (single-crystal X-ray diffraction methods) are conveniently compared. © 2011 The Royal Society of Chemistry.

Ultra high molecular weight polyethylene (UHMWPE) is a polymer that is widely used in industrial and orthopaedic applications. In this work, pristine multiwalled carbon nanotubes (MWCNTs) were incorporated into UHMWPE in different concentrations (1, 3 and 5. wt.%) using a ball milling process. UHMWPE/MWCNT nanocomposites were gamma irradiated at 90. kGy to improve the interaction between MWCNTs and the polymer matrix. Structural, thermal and mechanical characterizations were conducted by means of transmission electron microscopy (TEM), Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA) and uniaxial tensile techniques. Gamma irradiation produced an increase in the melting temperature, crystallinity and temperature of maximum decomposition rate. The irradiation produced a 38% decrease in the toughness of neat UHMWPE. The incorporation of MWCNTs did not significantly affect the melting point of the neat UHMWPE but decreased the degree of crystallinity of the raw UHMWPE, which was related to a reduction in the UHMWPE lamellar density. An increase in thermal stability was also observed for the nanocomposites compared to neat UHMWPE. The tensile tests showed a 38% increase in the Young's modulus in the reinforced nanocomposites and a small decrease in toughness (5%). Gamma irradiation of the nanocomposites increased crystallinity, which was related to an increased lamellar thickness, and also improved their thermal stability. The Young's modulus increased by up to 71% for irradiated nanocomposites and their toughness showed no significant changes in comparison with the non-irradiated nanocomposites. The incorporation of MWCNTs reduced the negative effects of irradiation and compensated for the reduction in toughness. This fact might be attributed to the radical scavenger behaviour of the MWNT as was proved by Electron Spin Resonance (ESR) detection of the radiation-induced radicals. © 2010 Elsevier Ltd.

Three pyridine 2,3-dicarboxylate complexes have been synthesized and characterized by IR spectroscopy, thermal analysis and single crystal X-ray diffractometry. Their magnetic properties have also been studied by EPR and magnetisation measurements. The decomposition of such complexes in air leads to the generation of mixed metal oxides, as confirmed by powder X-ray diffraction.

We show that a chemically engineered structural asymmetry in [Tb2] molecular clusters renders the two weakly coupled Tb3+ spin qubits magnetically inequivalent. The magnetic energy level spectrum of these molecules meets then all conditions needed to realize a universal CNOT quantum gate. A proposal to realize a SWAP gate within the same molecule is also discussed. Electronic paramagnetic resonance experiments confirm that CNOT and SWAP transitions are not forbidden.

A study of the hydrogen hyperfine couplings of the Mn-cluster of the oxygen-evolving complex of Photosystem II in the S2 state of the Kok cycle by means of hyperfine sublevel correlation spectroscopy was achieved. Features corresponding to hyperfine interaction of at least two hydrogen nuclei were detected. Combining our results with previous ENDOR data, hyperfine constants were determined, and by using a model for the structure and electronic spin state of the Mn-cluster, relevant structural information of the S2 state was obtained. This new information can be used for improving our knowledge about the structure and function of the Mn-cluster during light-driven water oxidation reaction in oxygenic photosynthetic organisms.

Homoleptic derivatives of formula [M(C(6)X(5))(n)](z-) (X = F, Cl) have been prepared and isolated for every first-row transition metal as well as for several of the heavier ones. The stoichiometry attained in each case (n ranging between 2 and 6) can be understood considering the tendency of a given metal ion to compensate its coordinative and electronic unsaturation, while not incurring severe interligand repulsive effects. The molecular structures associated with each stoichiometry seem, in turn, to be governed by electronic rather than steric factors. Most of these [M(C(6)X(5))(n)](z-) compounds are unsaturated, open-shell organometallic species, not fulfilling the 18-electron (or Effective Atomic Number) rule. This behaviour can be attributed to the absence of pi stabilising ligands (such as CO, phosphines, alkenes, alkynes, a variety of substituted aromatic rings, and so on) which are otherwise ubiquitous in organotransition-metal chemistry. The magnetic properties of the [M(C(6)X(5))(n)](z-) species have been determined by EPR spectroscopy and/or bulk magnetisation measurements. Excellent correlation between molecular geometry and magnetic properties (whether diamagnetic or paramagnetic) has been observed. Many of these compounds undergo chemically- or electrochemically-induced electron exchange processes. These redox reactions proceed without alteration in the stoichiometry of the [M(C(6)X(5))(n)](z-) compound, but usually involve a sharp change in the molecular geometry according to the different electron configuration of the interrelated species.